The invention relates to a method for examining a sample.
Imaging the surface of a sample using a scanning tunneling microscope (STM) is known from Binnig and Rohrer (G. Binnig, H. Rohrer (1987). Reviews of Modern Physics 59, 615).
The theory of scanning tunneling microscopy is known. During measurement with a scanning tunneling microscope, an electrically conductive tip or needle of the microscope is systematically moved in a grid pattern over the examination object, which is likewise conductive. The tip and the object surface are not in electrical contact during this process and, due to the insulating medium therebetween, e.g. air or vacuum, no current flow takes place in the macroscopic gap. However, if the tip approaches the surface at atomic orders of magnitude, the quantum mechanical states of the electrons (orbitals) of the surface and of the tip are superposed on one another, so that an exchange of electrons takes place with a probability greater than zero, which leads to a tunneling current by means of a tunneling effect, when a low voltage is applied. This tunneling current is highly sensitive to the smallest changes in distance, since the intensity is inversely exponentially proportional to the distance.
Various measurement modes for scanning tunneling microscopy are known. In the case of measurement at a constant height, the height of the tip is kept constant and the tunneling current varies in the course of scanning. This mode allows rapid scanning of the surface, but with an increased risk of breakage of the needle resulting from large changes in the structure of the sample. In a measurement mode with a constant tunneling current, the tunneling current is kept constant by a control circuit and the tip follows the surface. The resolution is high in this method, with the electronic structure of the surface being imaged on an atomic scale.
It is known from Tersoff and Hamann (J. Tersoff and D. R. Hamann (1985). Physical Review B 31, 805) that, by means of scanning tunneling microscopy, the local density of states (LDOS) in the region of the valence electrons is imaged, which is chemically unspecific, with structures that can extend over several atoms.
There have been various attempts to further provide the scanning tunneling microscope with a chemical sensitivity. For instance, the use of the inelastic tunneling process as a possibility for local, spatially resolved vibrational spectroscopy is known from Stipe et al. (B. C. Stipe, M. A. Rezaei and W. Ho (1998). Science 280, 1732).
The use of the optical luminescence properties of molecules or nanostructures for the spectroscopic identification thereof is known from Qiu et al. (X. H. Qiu, G. V. Nazin and W. Ho (2003) Science 299, 542).
Disadvantageously, none of these known methods allow for atomic-geometric/chemical imaging of the surface structure for a sufficiently diverse range of samples. Regardless of the measurement mode, conventional scanning tunneling microscopy only allows imaging of the chemical, or of the atomic-geometric structure of an examination object in special cases.
Another disadvantage of scanning tunneling microscopy is the lack of chemical sensitivity. This is to say that the method does not allow for identification of chemical species, and thus while it is possible to image molecular objects and surface structures to less than one angstrom in the lateral size range, it is usually not possible to identify them chemically or in any other way. The scanning tunneling microscope therefore does not give a clearly identifiable fingerprint of an adsorbed molecular object or of atomic surface structures. The reason for this shortcoming is the known conventional imaging mechanism of the scanning tunneling microscope.